A structure of the photomultiplier is examplified in Japanese Patent Laid-Open Publication No. 291654/1990 and is shown in FIG. 1.
The photomultiplier of FIG. 1 is of the so-called head-on type. In a glass tube 101 there are provided a photocathode 103 on an inside wall thereof, a focusing electrode 102, dynodes 104.about.113, and anodes 114. The voltage distribution of 350.about.1200 V which is increased toward the anodes 114 is applied to the dynodes 104.about.113. A pole electrode 115 is disposed between the first dynode 104 and the second dynode 105 for accelerating secondary electrons generated by the first dynode 104. A voltage sufficiently higher than that applied to the first dynode 104 (e.g., the same voltage as that applied to the fourth anode 107) is applied to the pole electrode 115.
When light is incident on a photocathode 103, photoelectrons are liberated. These photoelectrons are gathered to the focusing electrode 102 and sent to the first dynode 104. In the first dynode 104, secondary electrons are liberated by these photoelectrons and sent to the second dynode 105. The thus-generated secondary electrons at each of the following dynodes 105.about.113 are sent sequentially to its next dynode to be multiplied (cascade-multiplied), and multiplied photoelectrons are taken out finally at the anodes 114.
In the photoelectric multiplier of FIG. 1, a pole electrode 115 is disposed behind the third dynode 106, and the pole electrode 115 has a higher potential than the third dynode 106. Because of the presence of the pole electrode 115 at such position, which has a higher potential than the third dynode 106, an equipotential line E is bulged toward the first dynode 104. Because of such distribution of the equipotential line E, the secondary electrons emitted from the first dynode 104 are more accelerated when they transit toward the second dynode 105. Consequently an electron transit time of the emitted secondary electrons as a whole is shortened, whereby a spread of the electron transit time is relatively decreased.
In the acceleration of the secondary electrons by the above-described pole electrode 115, secondary electrons generated near the pole electrode 115 behind the dynode 104 are more accelerated. But secondary electrons emitted remote from the pole electrode 115 are less accelerated because their orbits are spaced from tile pole electrode 115. Consequently spreads (TTS's) of electron transit times cannot be sufficiently suppressed. As high-speed very feeble light pulse measurement, such as fluorescence lifetime measurement, time-resolved spectroscopy, etc., has been recently improved, photomultipliers having better transient response characteristics are needed.